Aquatic animal harvesting apparatuses such as fishing and trawling nets traditionally comprise a net that is towed under water by a towing vessel such as a boat. The nets comprise a mouth, lengthener, and a trailing ‘cod end’.
Traditionally nets are designed to be high strength and to have high porosity to water. During towing, a large volume of water flows through the entire length of the net, and aquatic animals are transported by the flow towards the trailing ‘cod end’ of the net. The animals are either retained in the cod-end or swept through the openings in the lengthener modules or cod-end mesh. The size of the aquatic animals captured depends on the grade of netting or mesh in the net; aquatic animals smaller than the net apertures are generally able to escape through the net.
The resistance of the net as it is towed through the water produces turbulence within the net that acts to tire the animals. These internal water flow patterns change with the volume of retained catch. Exhausted animals are tumbled constantly against each other and against the mesh. Even organisms much smaller than the mesh are often damaged through contact with the mesh strands or with other organisms. The contact between animals increases as the net fills. As the cod end fills with animals, a pressure wave may be created in front of the cod end, forcing animals into the net ahead of the cod end. This often causes the fish to become stuck or meshed in the net. As well as being damaging, meshed animals are labour intensive to remove.
Trawl nets most commonly comprise a diamond net structure, but some nets comprise a square mesh. Diamond meshes tend to collapse when they are empty or ‘neck’ down under the tension caused by a bolus of fish in the cod end of the net. When the meshes collapse, small fish are unable to escape, increasing the amount of by-catch. Square meshes are advantageous because they are more dimensionally stable under tension but tend to be expensive and are mechanically less forgiving that diamond mesh structures.
Even aquatic animals that are eventually ejected through conventional mesh systems often sustain un-survivable damage and levels of exhaustion.
Generally the longer a traditional net is towed through the water, the greater the damage sustained to the captured animals. Therefore, traditional nets cannot retain fish or other aquatic animals for extended periods of time in good condition and the nets must be emptied frequently.
When a traditional net is hauled out of the water and onto a boat, the fish may be further crushed against each other and the edge of the boat as the water drains from the net, exacerbating damage to the catch. This tissue damage can limit the utility and value of organisms caught. It is not uncommon for more than 50% of a catch to be unwanted or discarded in some fisheries. The impacting of the animals with each other also causes stress to the captured animals. This stress is undesirable as it causes autolytic spoilage, reducing the quality of the catch. It is known in meat processing that minimising stress to animals before slaughter improves the quality of the meat.
In addition, when a catch is brought on board in a traditional net, the catch is exposed through the net apertures. Detritus from the catch escapes through the net, attracting predators and scavengers such as birds, seals, sea lions, sharks and fur seals.
Attempts have been made to enhance the selectivity of trawl nets using rigid or flexible gratings or grids. These grids generally have the advantage that they do not collapse under tension, but they do not reduce inter-animal or animal-net contact and so do not reduce the damage to the catch.
WO 2008/064939 describes a trawl netting arrangement in which the minimum size of the fish caught is determined by the grade of the netting in the cod-end. The cod end portion contains escapements for species discrimination. The net contains internal ramps to direct fish out of the escapements and to increase the water flow rate near the escapements. There is a small reduction in flow rate in the netting arrangement drops from 100% of the tow speed at the mouth of the tapered trawl portion, to 60% at the end of the cod portion. Hydrodynamic kites are used to keep the net expanded.
Again in this arrangement, fish not ejected through the escapements remain in the net and may be forced against the end of the net. As the net fills up with fish, the fish are forced against each other and against the net. Again, when the net is hauled out of the water and onto a boat, the fish may be further crushed against each other.
WO 2004/032616 describes a trawl net having an impermeable closed end, for use in mid or bottom trawling. The trawl net is made up of a cylindrical netting portion attached to a standard trawl portion. At the end of the netting portion is an impermeable, closed cylindrical end, held in shape by external rings. While this impermeable end section may keep captured fish in a pool of water as the net is raised onto the ship's deck, the apparatus would have a significant flow rate of water back to the closed cylindrical end, which would cause fish to be forced against the netting portion in front of the impermeable closed end during trawling.
U.S. Pat. No. 6,883,265 describes a bag for transporting live fish that have already been harvested. The bag is towed along on or near the surface of a body of water. The bag contains a cylindrical tubular section made from a water-impermeable material. The bag has a mesh region covering the entrance and exit of the bag. Due to the leading and trailing mesh regions of the bag, that bag would not be suited to harvesting of aquatic animals. The bag is designed to keep fish contained during transportation. The bag also requires bag rings or other devices to keep the bag expanded. The flow rate of water through the bag is constant along the length of the bag.
U.S. Pat. No. 2,721,411 describes a trawl net with an open trailing end that feeds into an attached, rigid container with substantially impermeable walls for collecting and towing the captured fish. A flexible impervious flap is provided at the mouth of the container, which lowers when the container is full to prevent fish escaping. The rigid container contains a number of apertures sized to facilitate some water ‘sluicing’ through the container. Those apertures would be prone to blocking, and the rigid apparatus would be cumbersome and dangerous to handle onboard a marine vessel. In the net in U.S. Pat. No. 2,721,411 there is likely to be a significant flow of water through the net adjacent the entry to the container. That flow would force fish against the netting, damaging the catch.
There is therefore a need for an apparatus and method that enable aquatic animals to be harvested, while minimising both physical damage to the aquatic animals and the stress induced in the harvesting process to improve the quality of the aquatic animals that are harvested.
In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents or such sources of information is not to be construed as an admission that such documents or such sources of information, in any jurisdiction, are prior art or form part of the common general knowledge in the art.
It is an object of at least a preferred embodiment of the present invention to provide an apparatus and method of harvesting aquatic animals that addresses at least one of the abovementioned disadvantages, and/or to at least provide the public with a useful choice.